The present invention relates to calcined kaolin products, their production, and use. More specifically, the invention relates to calcined kaolin products having an apparent mean particle size of from about 0.5 xcexcm to about 10 xcexcm, and a multimodal particle size distribution defined herein. The calcined kaolin products of the invention possess a combination of physical and applied properties not previously seen together in calcined kaolin containing pigments. The products according to the invention find use as flattening (or matting) agents, and as fillers or extenders in paint and coating compositions. More generally, the inventive products may be used wherever calcined kaolins are used.
Particulate kaolins exist naturally in the hydrous form. In this form, kaolinite minerals form crystal structures that are linked together by hydroxyl containing moieties. Particulate kaolins may be converted to a calcined form by thermal processes. Such processes result in a dehydroxylation of the kaolin and an aggregation of the particles, and convert the crystal structure to an amorphous form.
Calcined kaolins inure benefits to application composition, such as improved opacity, compared with other pigments, e.g. calcium carbonate and hydrous kaolins. Accordingly, calcined kaolins find widespread use as pigments in paints, plastics, rubbers, sealants, ceramics, cementitious products, and other application compositions. Specifically, calcined kaolins confer desirable physical and optical (applied) properties to such compositions. As flattening (or matting) agents, they help to control the gloss and sheen of the surfaces of the substrates to which they are applied. As opacifiers, they impart brightness, whiteness, and other desirable optical properties. As extenders, they allow partial replacement of titanium dioxide and other more expensive pigments with minimal loss of whiteness or opacity.
Generally, the properties of kaolin pigments are influenced by the morphology of the particles that make up the pigment. Kaolin particle morphology for both hydrous and calcined kaolins is influenced by such factors as the size (expressed in terms of particle size distribution, or PSD, and median particle size), shape, and texture of the individual particles and of agglomerates thereof. Thus, kaolin pigments having a large percentage of small, or fine, calcined kaolin particles and a narrow PSD are known to make good opacifiers. See, e.g., Berube et al., U.S. Pat. No. 5,011,534, and Fanselow et al., U.S. Pat. No. 3,586,523. In contrast, calcined kaolins having relatively large, or coarse, particle size are known to be useful in making good paint compositions. See, e.g., Sare et al., U.S. Pat. No. 6,103,005.
A need in the art exists for calcined kaolin pigments that possess both good optical and physical properties in a single pigment product, however. The present invention satisfies this need. It provides a pigment having the characteristics of a flattening agent and opacifying agent in a single product. The pigments of the invention possess other useful qualities as well.
The present invention provides calcined kaolin products that embody a combination of physical and optical properties not previously available in a single calcined kaolin composition. More specifically, the calcined kaolin products of the invention possess a unique combination of good flattening (or matting) characteristics and good optical characteristics compared to previously available calcined kaolin pigments. The inventive products also serve as extenders, allowing the partial replacement of expensive titanium dioxide pigments without unacceptable loss of opacity or tint strength.
The calcined kaolin products of the invention are suited for use in paint or coating compositions in which any one of these characteristics are desired. They are particularly suited for use as pigments in paint or coating compositions in which a combination of two or more of these characteristics are desired. The products of the invention may also be useful wherever kaolins are used, such as in making filled plastics, rubbers, sealants, cables, ceramic products, cementitious products, and paper products and paper coatings.
In one embodiment, the inventive product comprises a blend of at least one calcined kaolin component and at least one calcined or hydrous kaolin component. The resulting blend will have a median particle size of from about 0.5 xcexcm (micrometers or microns) to about 10 xcexcm, and a multimodal particle size distribution (PSD). The multimodal PSD, when measured using SEDIGRAPH 5100, will exhibit at least two distinguishable components (or modes) of the composition: at least one of the distinguishable components will correspond to a fine calcined or hydrous kaolin component and occur at a particle size less than about 1 xcexcm, and at least one other of the distinguishable components will correspond to a coarse calcined or hydrous kaolin component and occur at a particle size of greater than about 2 xcexcm.
The blended products of this embodiment may be made by a method comprising:
(a) obtaining a coarse calcined or hydrous kaolin having a mean particle size greater than about 2 xcexcm;
(b) obtaining a fine calcined or hydrous kaolin having a mean particle size less than about 1 xcexcm; provided that at least one of said components (a) or (b) comprise calcined kaolin, and
(c) blending the coarse kaolin of (a) and the fine kaolin of (b) to produce the pigment product.
The product of this method may be referred to as a blended product, reflecting the fact that at least one of the coarse or fine feeds has been calcined prior to blending.
In another embodiment, the inventive product comprises a co-calcined kaolin composition having median diameter particle size of from about 0.5 xcexcm to about 10 xcexcm and a multimodal particle size distribution. The multimodal particle size distribution, when measured using SEDIGRAPH 5100, will exhibit at least two distinguishable components (or modes) of the composition: at least one distinguishable component will correspond to a fine calcined component and occur at a particle size less than about 1 xcexcm, and at least one other distinguishable component will correspond to a coarse calcined component and occur at a particle size greater than about 2 xcexcm.
The calcined kaolin containing products of this embodiment may be made by blending a coarse hydrous kaolin and fine hydrous kaolin components and then calcining the blend. More specifically, the products of this embodiment may be made by a method comprising:
(a) obtaining a coarse hydrous kaolin having a mean particle size greater than about 2 xcexcm;
(b) obtaining a fine hydrous kaolin having a mean particle size less than about 1 xcexcm; and
(c) blending the coarse hydrous kaolin of (a) and the fine hydrous kaolin of (b); and
(d) calcining the blend of (c) to produce the pigment product.
The product of this method may be referred to as a co-calcined product, reflecting the fact that the coarse and fine hydrous kaolin feeds have been blended prior to calcining.
The present invention also provides paint compositions and coated products comprising the pigments of the invention. More detailed expositions and examples of these and other aspects of the invention are provided below. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.